1. Technical Field of the Invention
The present invention relates in general to optical MEMS, and in particular, to micromachined interferometers.
2. Description of Related Art
Micro Electro-Mechanical Systems (MEMS) refers to the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through microfabrication technology. For example, the microelectronics are typically fabricated using an integrated circuit (IC) process, while the micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical components. MEMS devices are attractive candidates for use in spectroscopy, profilometry, environmental sensing, refractive index measurements (or material recognition), as well as several other sensor applications, due to their low cost, batch processing ability and compatibility with standard microelectronics. In addition, the small size of MEMS devices enables the integration of equipment incorporating MEMS devices, such as MEMS-based Michelson Interferometers, into mobile and hand held devices.
Moreover, MEMS technology, with its numerous actuation techniques, enables the realization of new functions and features of photonic devices, such as optical tunability and dynamic sensing applications. For example, by using MEMS actuation (electrostatic, magnetic or thermal) to control a movable mirror of a Michelson Interferometer, small displacements in the interferometer optical path length can be introduced, and consequently, a differential phase between the interfering beams can be obtained. The resulting differential phase can be used to measure the spectral response of the interferometer beam (e.g., using Fourier Transform Spectroscopy), the velocity of the moving mirror (e.g., using the Doppler Effect), or simply as an optical phase delay element.
MEMS-based Michelson Interferometers have been realized using Silicon On Insulator (SOI) technology, where beam splitting is performed using a thin wall of silicon (Si) or glass. However, the performance of current MEMS-based Michelson Interferometers is highly dependent on the thickness of the silicon or glass wall. In practice, the industrialization of such a device has been problematic, since repeatability of performance is highly sensitive to the fabrication process parameters. Another problem with using conventional thin silicon wall beam splitters is the spectral sensitivity of the “beam splitting ratio”, since the two Si/Air interfaces of the beam splitter contribute a parasitic Fabry-Pérot effect, thus modulating the power splitting ratio versus wavelength. This parasitic effect naturally degrades the performance of the Michelson Interferometer due to the noise introduced by operating in different wavelengths. Another problem in conventional beam splitters is the interference of the two separated beams from the two surfaces of the splitter. A silicon beam splitter was proposed in U.S. Pat. No. 4,632,553 to Vidring, et al., where the splitter was configured in a wedge shape. This wedge shape employed two splitting surfaces with different angles to avoid interference of the separated beams. However, although the wedge-shaped beam splitter solves the beam separation problem, it still suffers from the same problems of fabrication tolerance and parasitic dependence of “beam splitting ratio” on wavelength.
Therefore, there is a need for a more robust and more accurate micromachined interferometer. In particular, there is a need for a micromachined interferometer that exhibits a higher tolerance to the fabrication process, as well as a more stable spectral response of the beam splitting ratio.